Fiber reinforced composite products or articles have been made using a variety of processes and materials. One common type of composite article includes a polymeric matrix reinforced with a plurality of ceramic reinforcing fibers (e.g., glass fibers). Glass and other ceramic reinforcing fibers are commonly manufactured by supplying the molten ceramic material to a bushing, drawing fibers from the bushing, applying a chemical treatment, such as an aqueous-based system, so as to size the drawn fibers, and then gathering the sized fibers into a tow or strand.
In fabricating a composite part, the fibers in each of a plurality of fiber strands are often further chemically treated in an off-line impregnation process with either a thermoset resin, one or two part, or a thermoplastic resin. For example, in one such process a bundle or strand of glass fibers is impregnated with a heat curable thermosetting resin and then pulled through a heated pultrusion die, simultaneously curing the resin and making a composite part such as, for example, a ladder rail. Composite products have also been fabricated in a filament winding process by feeding the fiber strands through a bath of a heat curable thermosetting resin, winding the impregnated strand around a mandrel and curing the thermoset resin to form a filament wound part such as a pipe.
Composite articles made with a thermosetting polymer matrix can exhibit superior mechanical properties compared to composite articles made with a thermoplastic polymer matrix. This is particularly true when the composite article is exposed to high temperatures.
However, once the thermosetting resin is cured, the impregnated strand cannot be molded or reshaped in subsequent processing. This characteristic of a thermoset impregnated fiber strand has caused such strands to be formed into the desired composite article soon after the strand is impregnated. Typically, when a thermoset resin is used, the composite article has had to be formed in-line with the process of impregnating the fiber strand. In contrast, the ability of thermoplastic polymers to be remelted enables thermoplastic impregnated strands to be subsequently heated and molded or otherwise shaped into various articles or products in off-line processes. Thus, thermoplastic impregnated strands are typically easier to work with and more versatile.
Accordingly, there is a need in the art for a method for forming a fiber strand which exhibits the mechanical properties of a fiber strand coated with a thermosetting resin while also being easier to work with and more versatile than previous thermoset impregnated fiber strands. In particular, there is a need for a method of forming a thermoset coated fiber strand which can be formed into a composite article in a subsequent off-line forming operation and then fully cured.